Fix for failing asserts in HBoundsCheck code generation on x64: index register should be zero extended.

BUG=345820
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/180013002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19549 ce2b1a6d-e550-0410-aec6-3dcde31c8c00